As a method for producing a metal fine particle, there are known a physical method such as a vacuum vapor deposition method and a gas evaporation method; a chemical method such as a coprecipitation method and a hydrothermal method; and a mechanical method such as a pulverization method. Of these, the physical method is small in a problem of impurities remaining in a product fine particle and stable in quality as compared with other methods, and therefore, it is utilized for various materials and applications.
As to the vacuum vapor deposition method, in particular, there is a method called “continuous vacuum vapor deposition method onto active liquid surface”, which a raw material metal is heated and evaporated in vacuo, and a vapor of an atomic metal of the raw material is brought into contact with the surface of a liquid medium to generate a fine particle on the surface of the liquid medium, thereby producing a fine particle colloid dispersed in the liquid medium (for example, Patent Documents 1 and 2), and this method is known as a method for producing a high-quality metal fine particle colloid having a nanometer size. FIG. 1 is a diagrammatic view showing this method and a production apparatus of a metal fine particle colloid utilizing this. According to this method, a metal vapor 10 evaporated from a metal evaporation source 5 is brought into contact with a liquid medium film 9 in an upper part of a rotary vacuum chamber 2; and a metal fine particle 11 formed therein is formed into a colloid particle covered by a surfactant molecule on the spot, which is then put on the rotation of the rotary vacuum chamber 2 and transported into a bottom. At the same time, a new liquid medium film 9 is supplied into the upper part of the rotary vacuum chamber 2 from the bottom. By continuously performing this process, a liquid medium 3 of the bottom is changed to a stable colloid dispersion 12 in which a metal fine particle is dispersed in a high concentration.
On the other hand, the gas evaporation method (for example, Non-Patent Document 1) is a method in which after exhausting a container, by introducing a small amount of an inert gas such as an argon gas and heating and evaporating a raw material metal in the container while keeping the inside thereof in a reduced pressure state of the inert gas, a metal vapor is cooled due to a collision with the inert gas molecule in the vicinity of an evaporation source to form a metal fine particle; at the same time, a vapor of an organic solvent is supplied in the vicinity of the evaporation source; and the formed metal finer particle is guided into an exhaust pipe along with a gas flow of the organic solvent, deposited in a low-temperature part of the exhaust pipe and subsequently recovered. As compared with the previous vacuum vapor deposition method, this gas evaporation method is not high in efficiency and economy because supply of a large quantity of heat energy is necessary for evaporating the metal. But, the gas evaporation method can be utilized as a method capable of producing a high-quality metal fine particle.
However, in the foregoing production methods of a metal fine particle colloid, in case of producing a fine particle colloid of an alloy composed of plural kinds of elements, there was involved a problem that a composition of the alloy fine particle to be formed gradually changes. This problem is caused due to the following.
That is, first of all, in case of using an alloy composed of element components A and B as a raw material alloy, an alloy A1-XBX having a composition of an atomic ratio of the both of (1−X)/X is heated and melted in vacuo to form a homogeneous melt; when the temperature is further raised to vaporize it, the melt is radiated as a metal vapor in vacuo in a composition of an atomic ratio of (1−Y)/Y which is a ratio determined by vapor pressures inherent to the respective component elements; the element components respectively reach on a solid substrate or a liquid film of the liquid medium as referred to in this specification; and the A and B atoms are mutually condensed and solidified. When a condensation and solidification ratio is defined as (1−Z)/Z, an alloy fine particle having a composition of A1-ZBZ formed. This is expressed by the following expression.A1-XBX(s)→A1-XBX(l)→(1−Y)A(g)+YB(g)→A1-ZBZ(s)
Here, (s) stands for a solid state; (l) stands for a liquid state; and (g) stands for a gas state. Since it is considered that substantially all of atoms flying in vacuo are recovered, the relationship between Y and Z is Y=Z. Y does not depend upon X but depends upon the vapor pressures of the respective elements of the alloy. This is a so-called fractionation phenomenon and is a phenomenon which is utilized as a method for separation and purification using a different in boiling point of a multi-component solution such as a crude oil. When it is intended to evaporate an alloy of a fixed composition from a fixed amount of raw materials, evaporation preferentially occurs from a component having a higher vapor pressure; and as the raw materials are consumed, the composition ratio of the raw materials gradually changes, whereby a component having a lower vapor pressure finally remains. Accordingly, the alloy composition of a fine particle to be formed in the initial stage and the alloy composition of a fine particle to be formed in the final stage are largely different from each other so that it is difficult to obtain an alloy fine particle having a homogeneous composition.
As a countermeasure for avoiding such a problem, it may be considered to set up plural numbers of the metal element evaporation source 5. However, there are problems that the apparatus becomes large in size and complicated and that it is difficult to control the evaporation rate of each of the evaporation source.    Patent Document 1: JP-A-60-161490    Patent Document 2: JP-A-60-162704    Non-Patent Document 1: T. Suzuki and M. Oda, Proceedings of IMC 1996, Omiya, pp. 37, 1996